This disclosure relates to organic conducting films. It finds particular application in connection with an electrode formed from one or more electrically-conducting polymers and having two or more regions of different electrical conductivity. It is to be appreciated that the conductive films may find application in a variety of electronic devices, particularly opto-electronic devices, as well as in antistatic coatings and electromagnetic shielding applications.
Most opto-electronic devices, such as liquid crystal displays (LCDs), organic light-emitting devices (OLEDs), photovoltaic cells (PVs), and organic thin film transistors (OTFTs) employ one or more electrically conductive and transparent electrodes. Typically, various metal oxides, such as indium tin oxide (ITO) are used as electrodes, since they are highly conductive and transparent in the visible region. ITO, for example, can be vacuum vapor-deposited, sputtered, or pulsed laser deposited (PLO) onto glass or plastic substrates. ITO films with a surface resistance of less than 100 Ω/square and a high transparency of >90% can be easily obtained using these deposition methods. However, ITO has some disadvantages as an electrode material. Deposition techniques are carried out under vacuum and can be costly. Also, metal oxide films tend to be very brittle. They are generally not suitable for use with a flexible substrate since they tend to delaminate easily.
Recently, conducting polymers have attracted attention as a potential replacement for ITO in many electronic devices, especially for those using flexible substrates. This is primarily due to their good mechanical strength and ability to maintain their electrical and optical properties upon substrate flexing and bending (see, R. Paetzold, K. Heuser, D. Henseler, S. Roeger, G. Wittmann, and A. Winnacker, Appl. Phys. Lett., 82 (19), 3342, (2003)). Conducting polymers are particularly suitable, since their surface resistance does not seem to be affected either by sharp bending and/or by repeated bending cycles.
The application of conducting polymers as the electrode in polymer light-emitting devices (PLEDs) is disclosed, for example, in U.S. Pat. No. 5,766,515 to Jonas, et al. OLEDs fabricated on glass substrates using poly(3,4-ethylenedioxythiophene) (PEDOT) as the conducting polymer electrode and methoxyethylhexyloxy phenylenevinylene (MEH-PPV) as an emissive material are described. Molecular organic light-emitting diodes (MOLEDs) using an anode fabricated from a poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonic acid (PEDOT:PSS) conducting polymer and tris(8-hydroxyquinolinolato) aluminum (III) (Alq3) as an emissive layer have also been described (see W. H. Kim, A. J. Makinen, N. Nikolov, R. Shashidhar, H. Kim, and Z. H. Kafafi, Appl. Phys. Lett., 80, 3844, (2002); and W. H. Kim, G. P. Kushto, H. Kim, and Z. H., Kafafi, J. Polym. Sci. Part B: Polym. Phys., 41, 2471, (2003).) High external electroluminescence quantum efficiency and high luminance MOLEDs using a low sheet resistance conducting polymer anode based on high fluorescence and high electron mobility silole derivatives have also been developed. (see W. H. Kim, L. C. Palilis, M. Uchida, and Z. H. Kafafi, Polymer Electrodes for Flexible Organic Light-Emitting Devices, Chem. Mat. 16, 4681 (2004).
These references suggest that conducting polymers are promising candidates as anode materials and may eventually replace the most widely used ITO electrodes in many opto-electronic and other electronic devices, especially those fabricated on flexible substrates. Conducting polymers provide another advantage in applications such as OLEDs and OPVs due to their relatively high work function. The energy barrier for hole injection may be lowered due to the higher work function of the conducting polymers (˜5.0 eV) compared to that of ITO (˜4.7 eV). The X-ray and ultraviolet photoelectron spectroscopic (XPS and UPS) of PEDOT:PSS films have been studied. (see, R. Schlaf, H. Murata, and Z. H. Kafafi, J. Electron Spectrosc. Relat. Phenom. 120, 149 (2001).) The work function of the PEDOT:PSS films was measured to be 5.0±0.2 eV, regardless of the surface sheet resistance and the presence of various additives such as surfactants, polyalcohols and high boiling point solvents. This is significant for devices where the conducting polymers are used as an electrode since the device performance (such as driving voltage and efficiency) is largely dependent on the subtle change in the work function of the electrode. Therefore, the injection of holes from the electrode (anode) is not a limiting factor since the energy barrier between the anode and the organic layer is maintained.
However, the long-term stability of the conducting polymer electrode remains a problem. Some chemical degradation or electrical shorting is experienced under a high electric field, especially at the interface between the conducting polymer electrode and the organic layers. Therefore these devices tend to have poor operational stability and show rather low brightness and efficiency. Further, OLEDs fabricated using conducting polymer electrodes show very high leakage current that results in a low rectification ratio. A high ratio is beneficial for high resolution matrix displays where cross-talk between adjacent lines has to be avoided.
FIG. 1 shows the typical current density-voltage-luminance characteristics of an OLED based on the hole transporter, N,N′-Bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine (α-NPB) and Alq3 as the electron transporter/emitter, fabricated using a conducting polymer as an anode and a Mg:Ag alloy as a cathode. The device shows symmetric current-voltage characteristics in forward and reverse bias and does not exhibit a diode behavior. A low rectification ratio of about 1 over the range of ±5V is observed. The device also shows a rather low luminance especially at higher voltage. These are common characteristics for OLEDs using the conducting polymer anode that are caused by the leakage current.
There remains a need for a conductive polymer electrode which addresses these problems.